Photoelectron Spectroscopy and Ab initio Calculations of Peroxy Form of SO4- Anion

A photoelectron spectrum is reported for gas-phase SO 4 - formed in an electron-impact ionized free jet c SO 2 (1%)/O 2 (10%)/Ar. The vertical detachment energy (VDE) of the SO 4 - species is determined to be 3.7 ± 0.02 eV, which is significantly smaller than the VDE value reported for the sulfur-centered form of SO 4 extracted from an Na 2 S 2 O 8 solution. With the aid of MP2/6-311+G(2df) calculations, the newly formed specie is identified as a peroxy form of SO 4 - , which is well described as OOSO 2 - (C 1 , 2 A) having an S-O linkag between the O 2 and SO 2 units. The electronic properties of OOSO 2 - are characterized by the singly occupied molecular orbital (SOMO) constructed by an out-of-phase superposition of the 1a 2 orbital of SO 2 and th half-filled 2pπ g * of O 2 - . On the basis of these findings, a possible formation mechanism of OOSO 2 - is discusse in terms of a "solvent-mediated" addition reaction, where O 2 - stabilized by solvation attacks SO 2 while retainin the 2pπ g * excess electron.